This invention relates to a method of forming a spiral bevel gear, more particularly a method which makes it possible to form a spiral bevel gear having a small pitch angle (e.g. 45.degree. or less) by powder metallurgy, and a powder molding device.
Conventional sintered spiral bevel gears molded from powder had a large pitch angle. This is for the following reasons. As shown in FIG. 3, in the case of spiral bevel gears having a pitch angle .theta. not less than 45.degree., the entire tooth flanks appear in a plan view as seen from the apex side of the cone of spiral bevel gear, so that compressed molded articles can be punched out of dies without encountering any problem even on an ordinary powder molding device having its upper and lower punches and die nonrotatably supported.
But in the case of a spiral bevel gear having a pitch angle .theta.&lt;45.degree. as shown in FIG. 4, the left tooth flank LF or right tooth flank RF are hidden as viewed in plan. Thus, if one tries to simply punch a molded article out of the die, the article would not come out because the molded spiral teeth interfere with the mold (having teeth for molding).
Thus, conventional sintered spiral bevel gears made by powder metallurgy were limited to ones having a pitch angle .theta.&gt;45.degree., while conventional spiral bevel gears with .theta.&lt;45.degree. were all made by cutting.
Powder metallurgical method is higher in mass-productivity and lower in the manufacturing cost than cutting method. Thus, it is desired to manufacture spiral bevel gears having tooth flanks not seen in plan by powder metallurgy.
An object of this invention is to provide a method which solves this problem.